1. FIELD OF THE INVENTION
This invention relates to a process for producing a ferromagnetic iron oxide which is useful as a material for magnetic recording media. More particularly, the invention relates to a process for producing a cobalt- and ferrous iron-containing ferromagnetic iron oxide having excellent magnetic properties such as high coercivity and a good distribution of coercivity.
2. DESCRIPTION OF THE PRIOR ART
In comparison with .gamma.-Fe.sub.2 O.sub.3 powders or magnetic iron oxide powders containing cobalt alone which have hitherto been generally used, the cobalt- and ferrous iron-containing ferromagnetic iron oxide powder has a markedly high coercivity and the magnetic recording media produced therefrom are capable of high-density recording and also possess excellent properties such as good sensibility in the high frequency region.
Various methods have recently been proposed for producing such cobalt- and ferrous iron-containing ferromagnetic iron oxide powder. For example, the following methods are typically known: (1) A cobalt salt solution is added to an aqueous slurry of .gamma.-Fe.sub.2 O.sub.3, then an alkali is added thereto to coat the particle surfaces of .gamma.-Fe.sub.2 O.sub.3 with a cobalt hydroxide and then an aqueous solution of a ferrous salt is added to the slurry (Japanese Patent Publication 48444/81); (2) The method of (1) is carried out under heating (Japanese Patent Laid-Open 104721/81); and (3) the .gamma.-Fe.sub.2 O.sub.3 particles are dispersed in a mixed aqueous solution of a ferrous salt and a cobalt salt and then an alkali is added thereto to effect simultaneous coating of both ferrous hydroxide and cobalt hydroxide on the surfaces of said .gamma.Fe.sub.2 O.sub.3 particles (Japanese Patent Publication 36751/77).
However, according to the above-mentioned methods (1) and (2), the obtained magnetic iron oxide powder, although it is improved in coercivity, is unsatisfactory in the distribution of coercivity and magnetic tapes produced by using such powder would prove to be poor in the switching field distribution, squareness, orientability and other properties. In the case of the method (3), the magnetic iron oxide particle surfaces tend to be roughened as they could be partly dissolved by the alkali, and also an epitaxial reaction advances rapidly to broaden the distribution of coercivity. Thus, the improvements over these methods have been required.